This invention relates to a process and apparatus for treating molten metals, e.g. molten aluminum, with particulate treating agents particularly for inclusion removal, and removal of non-metallic or metallic elements.
It has long been a practice within the aluminum industry to treat molten aluminum with particulate treating agents such as halide salts for inclusion removal and alkali metal removal. For instance, MgCl2 may be added for alkali removal and a mixture of NaCl, KCl and cryolite may be used for solids removal from the molten aluminum.
One such previous system is described in Venas et al. U.S. Pat. No. 5,413,315, where a particulate treating agent is injected together with a gas downwardly through a hollow drive shaft having a cone-shaped rotor on the bottom end thereof. The mixture of particulate treating agent and gas is fed into the cone-like zone and a mechanism is also provided for withdrawing excess gas from the cone-like zone. A main principal of this design appears to be achieving mixing of the particulate treating agent with the molten aluminum with little agitation.
Other methods of treating molten aluminum with particulate treating agents are described in Forberg et al. Canadian Application 2,272,976, laid-open Nov. 27, 1999. This document describes a number of rotors used for crucible processing. Some of these rotors are of the shearing type with projecting teeth. Treatment salt is added either through the rotor or separately adjacent the rotor.
Skibo et al. U.S. Pat. No. 6,106,588 describes another device with a toothed rotor for injecting particulate material into molten aluminum. However, this is designed for adding particulates of material such as silicon carbide or alumina which do not dissolve or melt within the molten aluminum. Accordingly, the invention is concerned with the creation of high shear regions to facilitate wetting of the particulate material which is by its nature difficult to wet.
British Patent 1,422,055 discloses an apparatus for injecting a powder into a molten metal in a crucible that comprises a lance with an angled tip. A salt is delivered to the end of the lance by a screw device and gas is used in sufficient quantity to keep the metal out of the lance tip.
Yet another system for treating molten aluminum is described in Provencher et al. U.S. Pat. No. 5,080,715 where a salt is injected in a vortex while a gas is injected by a shaft.
It is an object of the present invention to provide an improved process for adding particulate treating agents to a molten metal, such as molten aluminum.
It is a further object to provide such improved process in which a minimum amount of gas is introduced into the molten metal and a maximum contact between the treating agent and the molten metal is achieved.
This invention in its broadest aspect relates to a process for treating molten metal with a particulate treating agent. In this process, a melt of a metal is provided in a treatment vessel such as a ladle. A mixing impeller is positioned substantially below the surface of the molten metal. The impeller comprises a plate with a series of spaced blades extending from the surface of the plate. This impeller is adapted to provide high shear mixing with minimum vortex. While rotating the impeller on a substantially vertical axis, particulate treating agent is fed by way of an injection tube below the surface of the molten metal and into the region between the axis and periphery of the impeller. This causes a high shearing action in the region of the blades whereby the treating agent is quickly broken down into finely divided, at least partially molten droplets which are circulated within the molten metal.
Preferably the blades are located at the periphery of the plate which is circular and are oriented tangential to the edge of the plate, i.e. the long dimension of the blades lies on a tangent to the movement of the impeller plate.
The treating agent is fed as a dense phase feed accompanied by the minimum amount of gas sufficient only to maintain a clear flow of the treating agent and to prevent any molten metal from travelling into the end of the conduit delivering the particulate material. The gas is preferably an inert gas, such as argon, helium or nitrogen, and is fed into a closed reservoir for the treating agent.
It has been found that by careful placement of the inlet for the particulate treating agent relative to the impeller, the treating agent is very quickly broken down by the blades into finely divided droplets which disperse throughout the molten metal. By quickly breaking down the treating agent droplets in the vicinity of the impeller blades, the efficiency is greatly improved because the surface contact between the treating agent and the molten metal is greatly increased. Furthermore, because the amount of gas added is much lower than normally used, there is a decreased tendency for the treating agent to be carried by gas bubbles to the top of the molten metal without having served its treatment purposes.
A further aspect of the invention comprises an apparatus for carrying out the above process. This apparatus includes:
(a) a treatment vessel adapted to hold molten metal,
(b) an impeller mounted on the lower end of a drive shaft extending substantially vertically downwardly into the vessel, the impeller comprising a plate with a series of spaced blades extending from a surface of the plate and being adapted to provide high shear mixing of molten metal contained in the vessel with minimum vortex,
(c) injector means for feeding a particulate treating agent into a region between the axis of the drive shaft and the periphery of the impeller, and
(d) means for rotating said drive shaft and impeller whereby said high shear mixing is achieved.
In one preferred embodiment of the invention the peripheral impeller blades are directed upwardly and the treating agent is fed downwardly through an fixed injection tube to a region between the axis and the periphery of the impeller.
In a further preferred embodiment, the peripheral impeller blades are directed downwardly on the bottom face of the impeller plate and the impeller is mounted on a hollow, rotatable drive shaft with the treating agent being fed downwardly through the hollow shaft to emerge beneath the impeller in a region between the exit of the hollow drive shaft and the downwardly directed peripheral blades of the impeller.
Additional radially mounted stirring blades may be used to provide additional general mixing of the molten metal within the vessel. These radially mounted stirring blades may be mounted on the reverse face of the impeller plate from the position of the peripheral blades. When such radially mounted stirring blades are mounted on the upper surface of the impeller plate they must be of sufficiently small area that they do not create any significant vortex in the metal.
The control of the vortex can be achieved by controlling the cross-sectional area of the blades perpendicular to the movement of the blades. In particular, the ratio of the volume swept by the blades to the area of the impeller plate perpendicular to the axis of rotation should not exceed 0.06 metres. The ratio is preferably in the range 0.002 to 0.06 metres.
According to a further embodiment of the invention, radially mounted blades may be used in place of the peripherally mounted tangential blades to create the required high shearing action. These radial blades thus serve to provide both the high shearing action and general mixing. They must, of course, also be designed as above to not create any significant vortex.
The peripheral speed of travel of the blades together with the location of the injection of the particulate treating agent provides a very high intensity initial contact between the treating agent and the metal particularly in the region of the outer periphery of the blades. Thus, a very high shearing action is created which serves to generate finely divided droplets of the treating agent. The blades typically travel at a tangential or peripheral velocity (measured at the outer periphery of the blades) of about 5-20 m/sec. Preferably they travel at a tangential or peripheral velocity of at least 8 m/sec.
According to a preferred feature of this invention, the impeller is operated only for a short period of time, e.g. less than 5 minutes (more preferably less than 3 minutes), under high intensity shearing conditions to disperse fine salt droplets, followed by a longer period (up to 10 minutes more preferably up to 5 minutes) of slow or non-mixing while the dispersed salt droplets are permitted to react within the molten aluminum.